finding average of a percentile of rows in a matrix?

Question

ARS 2012-7-26

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Hi All,

In the below given matrix, I wish to calculate the mean/average of first 20% rows and subtract it from the mean of the last 20% rows.

x =
       1   2   3   4   5
       2   4   6   8  10
       3   6   9  12  15
       4   8  12  16  20
       5  10  15  20  25
       1   2   3   4   5
       2   4   6   8  10
       3   6   9  12  15
       4   8  12  16  20
       5  10  15  20  25

this is a sample matrix, so there can be 100 rows and 100 columns but I need to average the values in the first 20% and subtract it from the average of last 20%. No sorting is needed in my matrix. therefore, in the above matrix, I would need to average the first two rows and subtract the resultant from the average of the last two rows.

any help will be appreciated.

Regards,

AMD.

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Answer 1

Azzi Abdelmalek 2012-7-26

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编辑：Azzi Abdelmalek 2012-7-26

在 MATLAB Online 中打开

   n=size(x,1);n1=round(n/5);row_mean=mean(x(n-n1+1:end,:))-mean(x(1:n1,:))
   result_mean=mean(row_mean)

% you can use the result: row_mean wich is a line % or the result: result_mean which is the mean of the line row_mean

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ARS 2012-7-30

Hi Azzi,

Sorry for being late. Couldn't reply due to illness.

My data has NaNs and the answer results in a NaN. How can I avoid NaNs in this calculation.

Regards,

AMD.

Azzi Abdelmalek 2012-7-30

can we replace them with 0?

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Answer 2

Wayne King 2012-7-26

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编辑：Wayne King 2012-7-26

在 MATLAB Online 中打开

Assume A is your matrix.

numrows = round(0.2*size(A,1));
lastrowstart = size(A,1)-numrows+1;
lowermean = mean(A(1:numrows,:),2);
uppermean = mean(A(lastrowstart:end,:),2);
uppermean-lowermean

Or did you mean take the mean over all elements in the bottom 20% of rows and upper 20% of rows, so you end up with a single number?

If that is the case:

    numrows = round(0.2*size(A,1));
    lastrowstart = size(A,1)-numrows+1;
    lowermean = mean(mean(A(1:numrows,:),2));
    uppermean = mean(mean(A(lastrowstart:end,:),2));
    uppermean-lowermean

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Oleg Komarov 2012-7-30

Use nanmean instead of mean.

ARS 2012-7-31

Thanks.

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Answer 3

Azzi Abdelmalek 2012-7-30

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编辑：Azzi Abdelmalek 2012-7-30

在 MATLAB Online 中打开

%replacing nan by zero

x(find(isnan(x)))=0 % add this to replace nan by zero

% the previous code

   n=size(x,1);n1=round(n/5);row_mean=mean(x(n-n1+1:end,:))-mean(x(1:n1,:))
   result_mean=mean(row_mean)

%in case you want calculate the mean, ignoring nan , that means: if i have [1 3 nan 4]; the mean will not (1+3+0+4)/4, but (1+3+4)/3. in this case add this code

 ind=arrayfun(@(y) ~isnan(y),x)
 x(find(isnan(x)))=0;
 n=size(x,1);n1=round(n/5);
 row_mean1=sum(x(n-n1+1:end,:))./sum(ind(n-n1+1:end,:))-sum(x(1:n1,:))./sum(ind(1:n1,:))